To achieve a good result with tissue expansion for unilateral breast reconstruction, it is of the utmost importance to select an expander, the size of which will allow the surgeon to obtain a reasonable symmetry with the contralateral breast. In this paper, we described a C++ computer program able to provide (supplied the semicircumference and file projection of the contralateral breast) objective, quantitative data useful to aid in proper expander selection when performing tissue expansion for unilateral breast reconstruction.